1. Field of the Invention
This invention relates to pressure vessels used for the storage of gases, particularly compressed natural gas (CNG) above ground.
2. Description of the Related Art
The use of CNG as an alternative motor vehicle fuel is on the rise. Some of the reasons include the fact that the use of CNG is cleaner with fewer emissions producing up to 99% less carbon monoxide than gasoline with almost no sulphur dioxide, no particulates and 85% less reactive hydrocarbons than gasoline. In addition, it provides better fuel economy and costs less than gasoline. There is some indication that it will provide less vehicle maintenance. Natural gas vehicles are safer than gasoline powered vehicles, since natural gas requires a temperature of 1300.degree. fahrenheit (704.degree. centigrade) to ignite, whereas gasoline ignites at a lower temperature of 842.degree. fahrenheit (450.degree. centigrade). Being lighter than air, CNG quickly dissipates into the atmosphere in the event of a spill or leak. Natural gas is a domestic, readily available, abundant, natural resource. Many more vehicles each year are being converted to run on clean burning natural gas.
Increasing use of CNG has required the ability to supply convenient storage and availability to vehicles so converted to natural gas.
Pressure vessels used for the storage of gases have traditionally been expensive due to the time and labor intensive manufacturing processes. Conventional methods of manufacture have been by welding component parts together or forging of the vessel. Both of these methods are expensive and time consuming. As a result, a very costly pressure vessel is produced requiring long lead times for manufacture. In addition, pressure vessels of conventional construction are extremely heavy, thereby causing difficulties and added cost in handling and transportation.
Welding of component parts of a pressure vessel is accomplished by obtaining a piece of pipe of desired length and specifications and welding a forged hemispherical section on each end. Each hemispherical section would have an opening therein to allow for gas access. Welding produces a pressure vessel with seams that are a line of reduced strength of the vessel. In addition, welding is a very labor intensive process.
Difficulties arise in the welding process when two sections of differing thicknesses are welded together. Joining of this type may require additional machining of the pieces to produce a taper in order for a satisfactory weld to be obtained.
A pressure vessel may also be constructed by welding sections of differing shapes to one another. An example of this is disclosed in the Watter patent, U.S. Pat. No. 3,024,938. Such construction also produces a vessel containing seams therein.
An alternative conventional method of construction of pressure vessels is accomplished through forging at high temperatures. Such methods of manufacture are equally as labor intensive and time consuming as those that are welded.
In using the forging method, a section of pipe of a desired length is obtained. In this method, in order to produce the hemispherical heads of the pressure vessel, the pipe is forged at extremely high temperature and the ends of the pipe are swaged closed. Once this is completed, the entire vessel is heat treated. After heat treatment, the swaged closed ends of the pipe are machined. The resultant pressure vessel is then cleaned and tested according to applicable specifications. This manufacturing process produces a seamless vessel, however, the cost of such production are high due to the heating and machining requirements.
Therefore, a need in the industry exists for a pressure vessel that is capable of storage of compressed gases, such as compressed natural gas which requires no expensive forging or welding. A need also exists for a pressure vessel where the manufacture time is expedited over conventional methods. A further need in the industry exists which conforms to ASME specifications yet is not as heavy as conventional vessels.